关键词: 柑橘, 果实形态, 果壳膨胀应力, 裂果, 裂隙规律

Abstract:

【Objective】The occurrence rules of cracks in citrus fruits with different fruit shapes were fully investigated to provide a new perspective for understanding the swelling and cracking of citrus fruits caused by field water imbalance before harvest.【Method】Based on the hierarchical characteristics of citrus peel structure, the fruit shell was considered a closed pressure vessel. By using the stress analysis method for the pressure vessel wall, the distribution of shell swelling pressure and the occurrence rules of peel cracks in different types of fruit shapes were evaluated.【Result】The shape of fruit shells affected the distribution of swelling pressure and the occurrence rules of cracks. (1) While the fruit shape index FSI in oval fruit was <1, the stress at the top of the fruit shell (i.e., σ_Top) was>equatorial meridional stress (i.e., σ_Φ-Mid) and>equatorial circumferential stress (i.e., σ_θ-Mid), which explained why the cracks in the oval-shaped fruit were often observed at the top of the fruit, e.g., the case for the early maturing cultivar Citrus reticulata Satsuma. If the fruit shape index FSI was >1, the stress at the top of the fruit shell (i.e., σ_Top) was<equatorial meridional stress (i.e., σ_Φ-Mid) and<equatorial circumferential stress (i.e., σ_θ-Mid), which could be frequently found in the elongated oval-shaped fruit with many longitudinal cracks like in the cultivar C. sinensis Navelina Navel. As for the fruit shape index FSI was =1, the cracks in the nearly spherical fruit shell were often located at the thinnest part of the fruit shell, and the ratio of stress at the top of the fruit to equatorial stress was inversely proportional to the thickness of the corresponding part of the fruit shell (i.e., σ_Top/σ_Mid=d_Mid/d_Top). For example, the cultivar Spring-sweet Tangelo often had circular transverse cracks at the thinnest part of the fruit waist, while the cultivar C. sinensis Newhall Navel cracks started at the thinnest part of the navel top. (2) The concave part of the nearly heart-shaped pomelo fruit was prone to forming a stress concentration condition for swelling, showing a great overlap with the point of maximum swelling pressure at the top of the sand sac, which led to a regular cracking at the top of the fruit as observed in C. grandis Duweiwendan. (3) Over the period with a dramatic increase of water potential in the field, the volume swelling ratio and fruit cracking rate in high-sugar fruits were higher than those in low-sugar fruits.【Conclusion】The cracks in citrus peel started at the point where the shell had the highest swelling pressure, while the direction of crack extension was perpendicular to the direction of maximum stress. Moreover, the more severe the non-sphericity and the more significant the difference in fruit shell thickness, the less the swelling pressure distribution and the higher the cracking rate of the fruit. A higher cracking rate of fruits with high sugar content than that of fruits with low sugar content was probably attributed to the unexpected increase in water potential.

Key words: citrus, fruit shape, shell swelling pressure, fruit cracking, crack pattern